late 14c., mewet "silent, not speaking," from Old French muet "dumb, mute" (12c.), diminutive of mut, mo, from Latin mutus "silent, speechless, dumb," probably from imitative base *meue- (source also of Sanskrit mukah "dumb," Greek myein "to be shut," of the mouth). Form assimilated in 16c. to Latin mutus. The meaning "incapable of utterance, dumb" is by mid-15c.
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.